1. Field of the Invention
This invention relates to the control of fire hazards, and it is concerned with providing materials which may be used in the control of fire hazards, for example in the extinguishing of fires, and methods of controlling fire hazards.
2. Description of the Related Art.
The invention was made in connexion with research into the control of so-called "Class D" fires, and with the control of fire hazards due for example to the escape of molten metal from a containment vessel. Class D fires include those due to burning metals.
Any burning material can of course be the cause of a secondary fire, but the risks of a secondary fire are particularly acute in the case of burning of molten metals because of the high temperatures associated with those materials and the difficulties of extinguishing burning metal and/or reducing the temperature at the site of the fire hazard.
Early efforts to control Class D fires arose as a result of the use of magnesium incendiary devices during the Second World War, and research has continued because of the increasing industrial use of inflammable metals such as magnesium, aluminium, zirconium and titanium. In parallel, fire risks arise in the nuclear industry where use is made of uranium, thorium and plutonium all of which are combustible, and in nuclear reactors and other plants where use is made of a fluid metal heat exchange system containing for example sodium or an alloy of sodium and potassium. Sodium and sodium-potassium fires are recognised as being particularly difficult to control, and their combustion products are quite noxious.
Among materials which were used at an early stage for the control of Class D fires were sand and natural silicates. But these materials, particularly in their less pure form, are not very effective against metal fires, especially alkali metal fires. Also they usually tend to be hygroscopic, and this can lead to caking and can make proper application to the fire hazard difficult.
The use of carbon has been proposed. Recent studies have indicated that a suitable extinguishant for alkali metal fires is expanded graphite or graphite microspheres, but these are very expensive materials. Furthermore, the use of carbon generally is really a counsel of desperation: in effect, the idea is to burn the carbon in order to deprive the alkali metal of oxygen. This does little to reduce the possibility of starting a secondary fire. Other powder extinguishants for alkali metal fires which have been found effective include metal salts, for example a mixture of NaCl 20%, KCl 29% and BaCl.sub.2 51% by weight. These materials are rather expensive, but their use can be justified against alkali metal fires which cannot easily be controlled in any other way. However, their use for fighting alkaline earth metal fires is less easy to justify, although they may be quite effective for this purpose. For use in fighting alkaline earth metal fires, for example for fighting magnesium fires, it has been proposed to use potassium or ammonium salts, for example potassium chloride (KCl) and acid ammonium phosphate (NH.sub.4 PO.sub.4 H.sub.2). It has also been proposed, for example for the control of zirconium fires, to use a dry powder comprising ground unexpanded or partially expanded perlite because that material contains about 4% to 6% combined water which is released as steam on heating and acts as a foaming agent so that the perlite can form a foam barrier over the burning metal. However, it is not desirable to use materials which release water for controlling alkali metal fires.